The present invention relates to a method of measuring a distribution of internal refractive index for a preform of an optical fiber, and to a measuring unit for performing the method.
The optical fiber is important as an information medium supporting intelligent society, and performance of the optical fiber depends on a shape of the refractive index distribution (hereinafter xe2x80x9creactive index distributionxe2x80x9d) formed inside of the fiber. A measuring technique for inspecting the formed structure is essential to the precise control technique of obtaining a shape measurement of the reactive index distribution. In this case, it is hard to measure the inside of the optical fiber in detail and the measurement is large in error because of the extremely thin diameter of the fiber. Therefore, the fiber is ordinarily measured at a preforming step (optical fiber preform, hereinafter called preform) which is the intermediate product before wire drawing the optical fiber. The structure of the reactive index for the optical fiber is formed at the inside of the preform by a chemical vapor deposition (CVD) method, a vapor-phase axial deposition (VAD) method, and so on. As the structure of the preform and the structure of the optical fiber are similar in shape, it is considered that the result of measuring the preform can be applied for the optical fiber by reduction of the scale. The optical fiber is produced by softening the preform with heat and by drawing the optical fiber to provide it with a predetermined diameter.
In the conventional method for measuring reactive index of the optical fiber preform, deflection function is found by permeating the measured object with parallel rays or thin laser radiation and by measuring reflection of rays received by distribution of internal reactive index.
However, in the measured object being large in change of reactive index such as high NA (number of opening) optical fiber preform, angle of deflection of transmitted light becomes large so that the light does not come in sight of a detector. Stated otherwise, the ray being large in angle of deflection is hard to detect and leaks from measurement so that accuracy of measurement decreases sharply.
The present invention is done at the view of the above-mentioned problem of the prior art, and an object of the present invention is to provide a method of measuring distribution of an internal reactive index of an optical fiber preform enabling to measure distribution of reactive index of a high NA optical fiber preform and, to a measuring device for performing the method. Particularly, the present invention provides a new measuring technique applicable to preforms which are hard to measure by the conventional method.
In the method of measuring a distribution of internal reactive index for an optical fiber preform according to the present inventions, a picture image is located behind the optical fiber preform being cylindrical and having uneven distribution of reactive index at inside of the material, and a deflected picture of said picture image is photographed through the optical fiber preform. By analyzing the deflected picture of said picture image, a deflection function is found, and a distribution of the reactive index of the optical fiber preform is found based on the deflection function.
In this case, it is desirable that the deflection function is found by a method of ray tracing from the deflected picture of the picture image.
It is desirable to light the picture image with an incoherent light source.
It is desirable to have a pattern having position data in the picture image where the light generates for the picture image.
A binary picture image of right-angled triangle can be used for such picture image. In this case, it is desirable to find a border line of variable density picture image determining threshold values different partly adapting to condition of the picture image, to find entire the border line combining the found border line, and to find a distribution of deflection of the picture from the whole border line.
A binary picture image of an isosceles triangle or picture image which hue or wavelength thereof changes continuously can be used for the picture image.
In the measuring device of distribution of internal reactive index for the optical fiber preform according to the present invention, a picture image is located behind the optical fiber preform being cylindrical and a having uneven distribution of reactive index at inside of the material, and a deflected picture of said picture image is photographed through the optical fiber preform. By analyzing the deflected picture of said picture image, a deflection function is found, and distribution of reactive index of the optical fiber preform is found based on the deflection function. The measuring device is characterized by comprising a picture image of pattern having position data, an incoherent light source lighting the picture image, a supporting unit for supporting the optical fiber preform in front of the picture image, a photographing unit for photographing the deflected picture of said picture image through the optical fiber preform, and an arithmetic and logic unit for finding a deflection function from the photographed deflected picture and for finding a distribution of reactive index of the optical fiber preform based on the deflection function.
In this case, an incoherent light source comprises a light source and a scattering board located at the front of the light source. An electroluminescence board can be used.
The photographing unit is desirable to have a diaphragm for limiting aperture, and a mask or a space filter.
In the present invention, the picture image is located behind the optical fiber preform being cylindrical and having uneven distribution of reactive index at inside of the material, and the deflected picture of said picture image is photographed through the optical fiber preform. By analyzing the deflected picture of said picture image, a deflection function is found, and a distribution of reactive index of the optical fiber preform is found based on the deflection function. Therefore, a photographing picture image is possible so as to photograph the deflected picture if the preform is a high NA optical fiber preform and an angle of deflection of a permeating ray becomes large. Therefore, even high NA optical fiber preform difficult in measuring by the conventional method can measure the distribution of reactive index accurately, and that contributes to improvement of performance of the optical fiber so as to contribute to the development of intelligent society.